Data conversion method for displaying an image

ABSTRACT

A data conversion method for displaying an image is provided in which selection of a subframe expression for reducing pseudo contours is systematized, and the subframe expression is optimized automatically. The method comprises the steps of determining a light emission waveform in accordance with display frame data of plural frames containing the current frame and the previous frame, performing Fourier expansion of an error between the determined light emission waveform and a target light emission waveform defined by the original frame data corresponding to the determined light emission waveform, and setting the display frame data of the current frame so that a sum of error components with weights that are obtained by weighting each Fourier component.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a data conversion method fordisplaying an image with gradation by controlling a light emission timeper one frame and a display device that uses the method. The inventionis suitable for a plasma display panel (PDP).

[0003] A PDP has both a high speed property and a high resolutionnecessary for a large screen display device of a TV set or a monitordisplay of a computer. One of the tasks of developing such a PDP is toreduce pseudo contours in displaying a moving image.

[0004] 2. Description of the Prior Art

[0005] A half tone is reproduced in a PDP by setting the number ofdischarges of each cell (each display element) for one frame inaccordance with a gradation level. A color display is one type of thegradation display, and a display color is determined by combination ofluminance values of the three primary colors.

[0006] A gradation display method for a PDP is known, in which one frameis made of plural subframes having weights of luminance, and the totalnumber of discharges of one frame is set by combining lighting andnon-lighting of each subframe (referred to as a subframe expression). Ingeneral, conversion of a frame into subframes is performed by using aconversion table that is prepared in advance. Furthermore, in the caseof an interlace display, each field of a frame includes pluralsubfields, and each subfield is controlled for lighting. However, thelighting control is the same as that of a progressive display.

[0007] In a display using a light control of subframe unit, lightedsubframes and non-lighted subframes are mixed so that light emissionsoccur at discrete timings in the frame period. Thus, a pseudo contourcan be generated. A pseudo contour is a phenomenon in which an observersees light and shade different from the display contents, and can begenerated easily when a portion of an image having pixels of similargradation levels constituting a gentle gradation change moves in ascreen. For example, in a scene with a walking human body, a pseudocontour can occur in a face of the human.

[0008] Conventionally, a method of reducing pseudo contours is known inwhich the weighting is devised so that plural subframe expressions arepossible for a half tone, and an optimum subframe expression is selectedfor each gradation level by noting each frame. A basic rule ofoptimizing the subframe expression is to stabilize the light emissionbarycenter in a frame period regardless of the gradation level asdisclosed in Japanese unexamined patent publication No. 10-307561. Forexample, the light emission barycenter is set to be always in the middleof the frame period. If the light emission barycenter is constant, aninterval of the light emission barycenter between frames becomesconstant, so that a deviation of the light emission timing such as along period of low luminance can be eliminated.

[0009] Moreover, Japanese unexamined patent publication No. 11-224074discloses a method of selecting an optimum subframe expression, in whicha frame to be converted into subframes (referred to as a current frame)is given a subframe expression by referring to a subframe expression ofthe previous frame and considering the relationship between the previousframe and the current frame. This method can reduce pseudo contours moresecurely than the method of determining the subframe expression bynoting only the current frame.

[0010] Conventionally, it is necessary that a skilled person decides asubframe expression to be selected for each gradation level based on theperson's experience when making a conversion table for coordinating aframe and subframes in order to reduce pseudo contours substantially.Especially, if the relationship between the previous frame and thecurrent frame is considered as mentioned above, an optimum subframeexpression should be determined for each of 256² combinations ofgradation when the number of gradation N equals to 256, so a vast laboris necessary. In addition, if two or more previous frames should bereferred to, the number of combinations of gradation is up to N³. If aspecification is revised by increasing the number of gradation N orchanging the weighting, the bothersome job is necessary.

SUMMARY OF THE INVENTION

[0011] An object of the present invention is to regulate selection of asubframe expression for reducing pseudo contours, and to realizeoptimizing the subframe expression by an automatic process.

[0012] In the present invention, Fourier component of an error between alight emission waveform depending on a subframe expression and an ideallight emission waveform is evaluated, and a subframe expression havingthe minimum error is selected from options of the subframe expression.Since a time resolution of a human sense of sight has difficulty indiscriminating a higher order of Fourier component, the error isevaluated by weighting each order of the Fourier component.

[0013] In the evaluation of an error by Fourier expansion, a time rangeof the expansion can be set arbitrarily. Therefore, a period of adisplay frame can be different from a period of an original frame.Moreover, since an ideal waveform to be a target can be set arbitrarily,the target is not limited to a step waveform that indicates a change ofdiscrete target values simply, but can be a line graph waveformconnecting target values with lines or an envelope waveform connectingtarget values with a smooth curve. In other words, target values are notnecessarily constant in an original frame period, but can be altered inthe original frame period.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a block diagram of a display device according to thepresent invention.

[0015]FIG. 2 shows an example of a cell structure of a PDP.

[0016]FIG. 3 shows a scheme of dividing a frame.

[0017]FIG. 4 shows an example of a light emission pattern.

[0018]FIG. 5 shows a target light emission waveform of type A.

[0019]FIG. 6 shows a target light emission waveform of type A and thecorresponding light emission waveform.

[0020]FIG. 7 shows a target light emission waveform of type B.

[0021]FIG. 8 shows a target light emission waveform of type A when theframe period is different.

[0022]FIG. 9 shows a target light emission waveform of type B when theframe period is different.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] Hereinafter, the present invention will be explained more indetail with reference to embodiments and drawings.

[0024]FIG. 1 is a block diagram of a display device according to thepresent invention.

[0025] The display device 100 comprises a surface discharge type PDP 1including a display surface having m×n cells, and a drive unit 70 forcontrolling cells arranged in a matrix to emit light selectively. Thedisplay device 100 is used as a wall-hanging TV set or a monitor displayof a computer system.

[0026] PDP 1 has display electrodes constituting electrode pairs forgenerating display discharges arranged in parallel and addresselectrodes arranged to cross the display electrodes. The displayelectrode extends in the row direction (horizontal direction) of thescreen, and the address electrode extends in the column direction(vertical direction).

[0027] The drive unit 70 includes a controller 71, a power sourcecircuit 73, a data converting circuit 75, an X driver 81, a Y driver 85,and an A driver 87. The drive unit 70 is supplied with frame data Df,i.e., multivalue image data indicating luminance levels of red, greenand blue colors together with various synchronizing signals fromexternal equipment such as a TV tuner or a computer.

[0028] In a display including a PDP 1, an original frame of an inputimage is divided into a predetermined number M of subframes so as toreproduce gradation by binary control of lighting. The data convertingcircuit 75 converts the frame data Df into subframe data Dsf for thegradation display and transmits the data to the A driver 87. Thesubframe data Dsf are a set of display data for M screens containing onebit per cell, and the value of each bit indicates whether the cell ofthe corresponding subframe is to be lighted, more specifically whetheran address discharge is necessary. The data converting circuit 75includes a frame memory 76 for memorizing frame data Df of at least oneframe, a subframe memory 77 for memorizing subframe data Dsf of at leastone frame, and a table memory 78 for outputting subframe data Dsf in amethod of looking up. The table memory 78 is supplied with latest framedata Df, frame data Df delayed by the frame memory 76, and subframe dataDsf delayed by the subframe memory 77. When converting the frame data Dfof the k-th frame to be displayed into the subframe data Dsf, the framedata Df of the previous frame including the (k−1)th frame and thesubframe data Dsf are referred to for selecting an optimum subframeexpression. The data of the table memory 78 are set so that Fouriercomponent of an error from a target becomes the minimum according to thepresent invention. Furthermore, an arithmetic processor may be providedinstead of the table memory 78, so that an optimum subframe expressioncan be determined by Fourier operation responding to an input.

[0029]FIG. 2 shows an example of a cell structure of a PDP.

[0030] As shown in FIG. 2, the PDP 1 comprises a pair of substratestructures (each structure made of a substrate on which cell elementsare arranged) 10 and 20. On the inner side of a glass substrate 11 of afront substrate structure 10, a pair of display electrodes X and Y isarranged for reach row of the display surface ES having n rows and mcolumns. Each of the display electrodes X and Y includes a transparentconductive film 41 that forms a surface discharge gap and a metal film42 that is overlapped on the edge portion of the transparent conductivefilm 41. The display electrodes X and Y are covered with a dielectriclayer 17, which is coated with a protection film 18.

[0031] On the inner side of the rear glass substrate 21, the addresselectrodes A are arranged, one for a column. The address electrodes Aare covered with a dielectric layer 24. On the dielectric layer 24, apartition 29 having a height of approximately 150 μm is provided. Apattern of the partition is a stripe pattern that divides a dischargespace into columns. The surface of the dielectric layer 24 and the sideface of the partition 29 are covered with fluorescent material layers28R, 28G, and 28B for color display. Italic letters (R, G and B) in FIG.2 indicate light emission colors of the fluorescent materials. The colorarrangement has a repeating pattern of red, green and blue colors inwhich cells in each column have the same color. The fluorescent materiallayers 28R, 28G and 28B are excited locally by ultraviolet raysgenerated by a discharge gas and emit light.

[0032]FIG. 3 shows a scheme of dividing a frame. FIG. 4 shows an exampleof a light emission pattern.

[0033] In order to reproduce a color by gradation display for eachcolor, a frame is divided into e.g., twelve subframes. Namely, a frameis replaced with a set of twelve subframes sf1-sf12. Weighting isperformed for setting the display discharge of each subframe, so that aratio of luminance values of the subframes is approximately5:16:59:32:3:7:2:1:22:9:43:56. Combinations of lighting and non-lightingof each subframe can make 256 steps of luminance setting for each ofred, green and blue colors.

[0034] The display frame period Tf is divided into subframe periodsTsf1-Tsf12 assigned to the subframes. Each of the subframe periodTsf1-Tsf12 is divided into a preparation period TR for equalizing chargedistribution in the whole screen, an address period TA for forming anelectrification distribution corresponding to display contents, and adisplay period TS for sustaining the lighted state so as to ensure aluminance corresponding to a gradation level. Lengths of the preparationperiod TR and the address period TA are constant regardless of theweight of luminance, and a length of the display period TS is larger fora larger weight of luminance.

[0035] As shown in FIG. 4, in a display of the gradation level 126(=59+2+22+43), the subframe expression is selected for lighting foursubframes sf3, sf7, sf9 and sf11.

[0036] Hereinafter, a data conversion method for optimizing the subframeexpression will be explained.

EXAMPLE 1

[0037] Here, one cell is noted, and the relationship between the celland each of the surrounding cells is not considered.

[0038] The luminance level to be displayed is denoted by f_(k). Thevariable k indicates the number of frame. The target waveform is a stepwaveform shown in FIG. 5. The form in which a target value does notchange within one frame is called “type A”.

[0039] The light emission intensity of the i-th subframe in the k-thframe is denoted by η^(k) _(i), a start point of a display period isdenoted by α_(k) _(i), and an end point thereof is denoted by β^(k)_(i). A unit of the time axis is a frame period, and origins of α^(k)_(i) and β^(k) _(i) are set at the head of the k-th frame. Furthermore,concerning η^(k) _(i), all frames have the same subframe structure, andthe luminance level when only the i-th subframe is lighted is denoted byf_(SF) ^(k) _(i). Then, the luminance level f_(SP) ^(k) _(i) isstandardized by the following equation.

f _(SF) ^(k) _(i)=η^(k) _(i)(β^(k) _(i)−α^(k) _(i))  (1)

[0040] If the period of the display discharge does not change dependingon a subframe, η^(k) _(i) is also independent of a subframe and issubstantially a constant value. In addition, the subframe structure canbe different for each frame. The expansion into Fourier series isperformed in a period of successive L frames. A point on the time axishaving a unit of frame period is denoted by t, and the origin is set tothe head of 0-th frame. Then, a fundamental function system is expressedas follows. $\begin{matrix}\left\{ {\frac{1}{2},{\cos \frac{2{n\pi}\quad t}{L}},{\sin \frac{2{n\pi}\quad t}{L}}} \right\} & (2)\end{matrix}$

[0041] The same fundamental function system is used without depending ona period to be expanded. Here, n is a natural number. The light emissionpattern of subframes of the k-th frame is determined so that an errorbetween the light emission waveform and the target light emissionwaveform is minimized. Then, the error is evaluated by weightingcomponents of Fourier expansion of the difference between the lightemission waveform and the target light emission waveform in a periodthat is L frames before the k-th frame.

[0042] When the light emission waveform is denoted by φ(t) and thetarget light emission waveform is denoted by f(t), Fourier expansion ofan error in the period of L frames is derived by the following equation.$\begin{matrix}{{{\varphi (t)} - {f(t)}} = {\frac{a_{0}}{2} + {\sum\limits_{n = 1}^{\infty}\quad \left( {{a_{n}\cos \frac{2{n\pi}\quad t}{L}} + {b_{n}\sin \frac{2{n\pi}\quad t}{L}}} \right)}}} & (3)\end{matrix}$

[0043] Here, the coefficients are as follows. $\begin{matrix}{{a_{n} = {\frac{2}{L}{\int_{k - L + 1}^{k + 1}{\left( {{\varphi (t)} - {f(t)}} \right)\cos \frac{2{n\pi}\quad t}{L}\quad {t}\quad \left( {{n = 0},1,2,\ldots}\quad \right)}}}}{b_{n} = {\frac{2}{L}{\int_{k - L + 1}^{k + 1}{\left( {{\varphi (t)} - {f(t)}} \right)\sin \frac{2{n\pi}\quad t}{L}\quad {t}\quad \left( {{n = 1},2,\ldots}\quad \right)}}}}} & (4)\end{matrix}$

[0044] Since the fundamental function system is fixed, the integralperiod in the equation (4) can be divided into each frame period, andthe sum can be calculated later. The integral of each frame is definedas follows. $\begin{matrix}{{a_{n}^{k} = {\frac{2}{L}{\int_{k}^{k + 1}{\left( {{\varphi (t)} - {f(t)}} \right)\cos \frac{2{n\pi}\quad t}{L}\quad {t}\quad \left( {{n = 0},1,2,\ldots}\quad \right)}}}}{b_{n}^{k} = {\frac{2}{L}{\int_{k}^{k + 1}{\left( {{\varphi (t)} - {f(t)}} \right)\sin \frac{2{n\pi}\quad t}{L}\quad {t}\quad \left( {{n = 1},2,\ldots}\quad \right)}}}}} & (5)\end{matrix}$

[0045] Using the equations (5), the coefficients defined by theequations (4) are rewritten as follows. $\begin{matrix}{{a_{n} = {\sum\limits_{j = {k - L + 1}}^{k}\quad a_{n}^{j}}}{b_{n} = {\sum\limits_{j = {k - L + 1}}^{k}\quad b_{n}^{j}}}} & (6)\end{matrix}$

[0046] Next, the integrals of the equations (5) are calculated. First,the lighting pattern of subframes in k-th frame is denoted by δ^(k)(i).If the i-th subframe is lighted, δ^(k)(i)=1. If the i-th subframe is notlighted, δ^(k)(i)=0. In addition, a function S_(α,β)(t) is used that hasthe value “1” in the period from α to β and the value “0” in the otherperiod. Then, φ(t) in the period of k-th frame can be expressed asfollows.

[0047] Function:S_(α,β)(t) $\begin{matrix}{{\varphi (t)} = {\sum\limits_{i = 1}^{M_{k}}\quad {{\delta^{k}(i)}\eta_{i}^{k}{S_{{k + \alpha_{i}^{k}},{k + \beta_{i}^{k}}}(t)}}}} & (7)\end{matrix}$

[0048] Here, M_(k) is the total number of subframes in the k-th frame.In the k-th frame period, f(t) is expressed as follows.

f(t)=f _(k)  (8)

[0049] Therefore, the following equations are derived. $\begin{matrix}{{a_{0}^{k} = {{\frac{2}{L}{\sum\limits_{i = 1}^{M_{k}}\quad {{\delta^{k}(i)}{\eta_{i}^{k}\left( {\beta_{i}^{k} - \alpha_{i}^{k}} \right)}}}} - {\frac{2}{L}f_{k}}}}{a_{n}^{k} = {{\left( \frac{1}{n\pi} \right){\sum\limits_{i = 1}^{M_{k}}\quad {{\delta^{k}(i)}{\eta_{i}^{k}\left( {{\sin \frac{2{n\pi}}{L}\left( {k + \beta_{i}^{k}} \right)} - {\sin \frac{2{n\pi}}{L}\left( {k + \alpha_{i}^{k}} \right)}} \right)}}}} - {\left( \frac{1}{n\pi} \right){f_{k}\left( {{\sin \frac{2{n\pi}}{L}\left( {k + 1} \right)} - {\sin \frac{2{n\pi}}{L}k}} \right)}\quad \left( {{n = 1},2,\ldots}\quad \right)}}}{b_{n}^{k} = {{{- \left( \frac{1}{n\pi} \right)}{\sum\limits_{i = 1}^{M_{k}}\quad {{\delta^{k}(i)}{\eta_{i}^{k}\left( {{\cos \frac{2{n\pi}}{L}\left( {k + \beta_{i}^{k}} \right)} - {\cos \frac{2{n\pi}}{L}\left( {k + \alpha_{i}^{k}} \right)}} \right)}}}} + {\left( \frac{1}{n\pi} \right){f_{k}\left( {{\cos \frac{2{n\pi}}{L}\left( {k + 1} \right)} - {\cos \frac{2{n\pi}}{L}k}} \right)}\quad \left( {{n = 1},2,\ldots}\quad \right)}}}} & (9)\end{matrix}$

[0050] From the equations (9) and (6), Fourier coefficients areobtained.

[0051] Hereinafter, an error of the light emission distribution that issensed by human eyes is discussed. A sensitivity of human eyes (or aquantity proportional to the sensitivity) for each frequency of Fouriercomponents is denoted by ξ_(n). Then, the error with weight ξ_(n) of thelight emission waveform in the period of L frames that can be sensed byhuman eyes is as follows. $\begin{matrix}{{E_{h}(t)} = {{\xi_{0}\left( \frac{a_{0}}{2} \right)} + {\sum\limits_{n = 1}^{\infty}\quad {\xi_{n}\left( {{a_{n}\cos \frac{2{n\pi}\quad t}{L}} + {b_{n}\sin \frac{2{n\pi}\quad t}{L}}} \right)}}}} & (10)\end{matrix}$

[0052] A square average of this error within the period of L frames iscalculated as follows. $\begin{matrix}{E_{L} = {{\left( \xi_{0} \right)^{2}\left( \frac{a_{0}}{2} \right)^{2}} + {\sum\limits_{n = 1}^{\infty}\quad {\left( \xi_{n} \right)^{2}\left( {\left( a_{n} \right)^{2} + \left( b_{n} \right)^{2}} \right)}}}} & (11)\end{matrix}$

[0053] When the lighting pattern δ^(k)(i) of the k-th frame isdetermined, in the equation (11), other quantities than the lightingpattern of the k-th frame are known. The lighting pattern of the k-thframe is determined so that the error E_(L) with weight is minimized.The expression of E_(L) is organized by the unknown variable δ^(k)(i) tobe rewritten as follows. $\begin{matrix}{E_{L} = {{\sum\limits_{i = 1}^{M_{k}}\quad {G_{i}^{k}{\delta^{k}(i)}}} + {\sum\limits_{i < j}{H_{i,j}^{k}{\delta^{k}(i)}{\delta^{k}(j)}}} + Q_{k}}} & (12)\end{matrix}$

[0054] Here, G^(k) _(i), H^(k) _(i,j) and Q^(k) are known quantities asexpressed below. $\begin{matrix}{{{G_{i}^{k} = {{\left( \xi_{0} \right)^{2}\left( {{\frac{1}{L^{2}}\left( \eta_{i}^{k} \right)^{2}\left( S_{i}^{k} \right)} + {\frac{a_{0}^{\prime}}{L}\eta_{i}^{k}S_{i}^{k}}} \right)} + {\sum\limits_{n = 1}^{\infty}{\left( \xi_{n} \right)^{2}\begin{bmatrix}{2\left( \frac{1}{n\quad \pi} \right)^{2}\left( {{\eta_{i}^{k}}^{2}\left( {1 - {\cos \quad \frac{2n\quad \pi}{L}S_{i}^{k}} +} \right.} \right.} \\{4\left( \frac{1}{n\quad \pi} \right){\eta_{i}^{k}\begin{pmatrix}{{a_{n}^{\prime}\cos \quad \frac{2n\quad \pi}{L}\left( {k + P_{i}^{k}} \right)\sin \quad \frac{2n\quad \pi}{L}S_{i}^{k}} +} \\{b_{n}^{\prime}\sin \quad \frac{2n\quad \pi}{L}\left( {k + P_{i}^{k}} \right)\cos \quad \frac{2n\quad \pi}{L}S_{i}^{k}}\end{pmatrix}}}\end{bmatrix}}}}}H_{i,j}^{k} = {{2\left( \xi_{0} \right)^{2}\frac{1}{L^{2}}\eta_{i}^{k}\eta_{j}^{k}S_{i}^{k}S_{j}^{k}} + {\sum\limits_{n = 1}^{\infty}{8\left( \xi_{n} \right)^{2}\left( \frac{1}{n\quad \pi} \right)^{2}\eta_{i}^{k}\eta_{j}^{k} \times \left\lbrack \quad \begin{matrix}{{\cos \quad \frac{2n\quad \pi}{L}\left( {k + P_{i}^{k}} \right)\cos \quad \frac{2\quad n\quad \pi}{L}\left( {k + P_{j}^{k}} \right)\sin \quad \frac{2\quad n\quad \pi}{L}S_{i}^{k}\sin \quad \frac{2\pi \quad n}{L}S_{j}^{k}} +} \\{\sin \quad \frac{2n\quad \pi}{L}\left( {k + P_{i}^{k}} \right)\quad \sin \quad \frac{2n\quad \pi}{L}\left( {k + P_{j}^{k}} \right)\cos \quad \frac{2\quad n\quad \pi}{L}S_{i}^{k}\cos \quad \frac{2n\quad \pi}{L}S_{j}^{k}}\end{matrix} \right\rbrack}}}}{Q = {{\left( \xi_{0} \right)^{2}\left( \frac{a_{0}^{\prime}}{2} \right)^{2}} + {\sum\limits_{n = 1}^{\infty}{\left( \xi_{n} \right)^{2}\left( {\left( a_{n}^{\prime} \right)^{2} + \left( b_{n}^{\prime} \right)^{2}} \right)}}}}} & (13)\end{matrix}$

[0055] The coefficients are defined as follows. $\begin{matrix}{{P_{i}^{k} = {\frac{1}{2}\left( {\alpha_{i}^{k} + \beta_{i}^{k}} \right)}}{a_{0}^{\prime} = {{\sum\limits_{j = {k - L + 1}}^{k - 1}a_{0}^{j}} - {\frac{2}{L}f_{k}}}}{a_{n}^{\prime} = {{\sum\limits_{j = {k - L + 1}}^{k - 1}a_{n}^{j}} - {\left( \frac{1}{n\quad \pi} \right){f_{k}\left( {{\sin \quad \frac{2\quad n\quad \pi}{L}\left( {k + 1} \right)} - {\sin \quad \frac{2\quad n\quad \pi}{L}k}} \right)}}}}\left( {{n = 1},2,\ldots}\quad \right){b_{n}^{\prime} = {{\sum\limits_{j = {k - L + 1}}^{k - 1}b_{n}^{j}} - {\left( \frac{1}{n\quad \pi} \right){f_{k}\left( {{\cos \quad \frac{2\quad n\quad \pi}{L}\left( {k + 1} \right)} - {\cos \quad \frac{2n\quad \pi}{L}k}} \right)}}}}\left( {{n = 1},2,\ldots}\quad \right)} & (14)\end{matrix}$

[0056] Consequently, since the light emission pattern of a new frame isdetermined in accordance with the light emission pattern of the previousframe and display luminance, the relationship therebetween may becalculated beforehand to be a table.

[0057] As explained above, an error is evaluated not by a displaygradation level but by a display luminance. It is because that onedisplay gradation level can generate different luminance levelsdepending on a display load. If the variation of the display load is notsubstantially large, an error can be evaluated not by a waveform of thelight emission intensity but by a waveform of the gradation level(gradation waveform). In this case, in the equations explained above,φ(t), f(t), f_(k), f_(SF) ^(k) _(i) and η^(k) _(i) denote quantities ofthe gradation level. A relationship table for determining a new lightemission pattern is a table of the relationship between the lightemission pattern of the past frame and the display gradation level. Thisstructure can be adopted since it is expected that the rapid change ofthe display load does not occur frequently. This structure has anadvantage in that the relationship table can be compact. In addition,ξ_(n) can be set in an approximate manner. For example, for Fouriercomponent corresponding to a frequency above the flicker frequency thatcan be discriminated by human sense about the intensity variation, valueof ξ_(n) can be set as ξ_(n)=0. For Fourier component corresponding to afrequency below the flicker frequency, value of ξ_(n) can be set asξ_(n)=1. Since the flicker frequency is lowered for lower luminancelevel, ξ_(n) can be a function of the display luminance.

[0058] Moreover, a value above the flicker frequency is normallyselected for the frame frequency. Therefore, the value of ξ_(n) can beset to “0” for Fourier component corresponding to a frequency above theframe frequency and to “1” for Fourier component corresponding to afrequency below the same. More specifically, ξ_(n) is expressed asfollows.

ξ_(n)=1 (n≦L−1)

ξ_(n)=0 (n≦L)  (15)

[0059] The set value of the weight ξ_(n) is not limited to theabove-mentioned example. For example, a₀/2 of the error components is anerror of the gradation level. If a faithful reproduction of thegradation level is required, the value of ξ₀ is set large. In addition,if a particularly strict faithfulness of the reproduction of thegradation level is required, the light emission pattern is selected asfollows.

a ₀=0  (16)

[0060] In this case, the structure of the subframe is required to becapable of expressing any gradation level. If there are plural lightemission patterns that can express the same gradation level, the lightemission pattern that can minimize the error E_(L) is selected. Theintensity of one or more Fourier component is preferably low so thatpseudo contours and flickers can be reduced. If an error of thegradation level is permitted to a certain extent, under the conditiondefined by the expression (17), the light emission pattern can be sodetermined as to minimize the error E_(L)′ defined by the equation (18).

a ₀ ≦D  (17)

[0061] $\begin{matrix}{E_{L}^{\prime} = {\sum\limits_{n = 1}^{\infty}{\left( \xi_{n} \right)^{2}\left( {\left( a_{n} \right)^{2} + \left( b_{n} \right)^{2}} \right)}}} & (18)\end{matrix}$

[0062] In this case too, the weight ξ_(n) is set approximately to “0”for Fourier component above the flicker frequency and to “1” for Fouriercomponent below the same. In addition, a gradation permitted error D canbe a function of the display luminance, too. If the error of thegradation is permitted, options for selecting a light emission patternare increased so that pseudo contours and flickers can be reducedeasily. In addition, it is desirable that a user can select whether theconditions defined in expressions (16) and (17) are valid or not, andthat a user can adjust the weighting according to the user's preference.

[0063] If the condition of the equation (16) is valid, it is necessarythat all gradation levels of display data can be displayed. However, anerror of the gradation level is permitted in other cases, so thesubframe structure that can express all gradation levels is not alwaysnecessary. Moreover, the gradation level that can be expressed by acombination of light emission patterns of subframes is usually set to avalue of multiple of the minimum gradation level by an integer. However,it is unnecessary for the selection method of the light emission patternaccording to the present invention in which an error of the gradationlevel is permitted. Conventionally, when expressing a gradation levelthat cannot be expressed by a lighting pattern of subframes, an areagradation method or an interframe modulation method is utilized.However, according to the present invention, the light emission patternis determined by evaluating an error E_(L), so that the gradation levelto be a target can be automatically displayed without combining anothermethod.

[0064] Furthermore, in order to determine the subframe expression of thecurrent frame, the light emission pattern of the previous frame and thedisplay luminance level (or the display gradation level) are used.Therefore, the light emission pattern and the display luminance level(or the display gradation level) for each frame of at least (L-1) framesin the past should be memorized. After the subframe expression of thecurrent frame is determined, the light emission pattern and the displayluminance level of the frame are memorized, and old data that are notused for the later calculation are erased.

EXAMPLE 2

[0065] The light emission intensity distribution as shown in FIG. 6 is atarget in Example 1, while a line graph waveform as shown in FIG. 7 canbe the target light emission waveform. The form in which a target valuechanges within one frame is called “type B”. The waveform shown in FIG.7 is a primary interpolation waveform obtained by linear approximationof a target transition within a frame in accordance with a luminancelevel of each frame. This example is similar to Example 1 except forexpressions of Fourier coefficients.

f(t)=(f _(k+1) −f _(k))(t−k)+f _(k)  (19)

[0066] The expressions of Fourier components are as follows.$\begin{matrix}{{{a_{0}^{k} = {{\frac{2}{L}{\sum\limits_{i = 1}^{M}{{\delta^{k}(i)}{\eta_{i}^{k}\left( {\beta_{i}^{k} - \alpha_{i}^{k}} \right)}}}} - {\frac{1}{L}\left( {f_{k} + f_{k + 1}} \right)}}}{a_{n}^{k} = {{\left( \frac{1}{n\quad \pi} \right){\sum\limits_{i = 1}^{M}{{\delta^{k}(i)}{\eta_{i}^{k}\left( {{\sin \quad \frac{2\quad n\quad \pi}{L}\left( {k + \beta_{i}^{k}} \right)} - {\sin \quad \frac{2\quad n\quad \pi}{L}\left( {k + \alpha_{i}^{k}} \right)}} \right)}}}} - {\left( \frac{1}{n\quad \pi} \right)\left( {{f_{k + 1}\sin \quad \frac{2n\quad \pi}{L}\left( {k + 1} \right)} - {f_{k}\sin \quad \frac{2\quad n\quad \pi}{L}k}} \right)} - {\left( \frac{L}{2n^{2}\pi^{2}} \right)\left( {f_{k + 1} - f_{k}} \right)\quad \left( {{\cos \quad \frac{2n\quad \pi}{L}\left( {k + 1} \right)} - {\cos \quad \frac{2n\quad \pi}{L}k}} \right)}}}\left( {{n = 1},2,\ldots}\quad \right)\quad {b_{n}^{k} = {{{- \left( \frac{1}{n\quad \pi} \right)}{\sum\limits_{i = 1}^{M}{{\delta^{k}(i)}{\eta_{i}^{k}\left( {{\cos \quad \frac{2\quad n\quad \pi}{L}\left( {k + \beta_{i}^{k}} \right)} - {\cos \quad \frac{2\quad n\quad \pi}{L}\left( {k + \alpha_{i}^{k}} \right)}} \right)}}}} + {\left( \frac{1}{n\quad \pi} \right)\left( {{f_{k + 1}\cos \quad \frac{2n\quad \pi}{L}\left( {k + 1} \right)} - {f_{k}\cos \quad \frac{2\quad n\quad \pi}{L}k}} \right)} - {\left( \frac{L}{2n^{2}\pi^{2}} \right)\left( {f_{k + 1} - f_{k}} \right)\quad \left( {{\sin \quad \frac{2n\quad \pi}{L}\left( {k + 1} \right)} - {\sin \quad \frac{2n\quad \pi}{L}k}} \right)}}}\left( {{n = 1},2,\ldots}\quad \right)}\quad} & (20)\end{matrix}$

[0067] Though the expression (13) does not change, a part of theexpression (14) changes as the expression of Fourier coefficientschanges. $\begin{matrix}{{{a_{0}^{\prime} = {{\sum\limits_{j = {k - L + 1}}^{k - 1}a_{0}^{j}} - {\frac{1}{L}\left( {f_{k} + f_{k + 1}} \right)}}}{a_{n}^{\prime} = {{\sum\limits_{j = {k - L + 1}}^{k - 1}a_{n}^{j}} - {\left( \frac{1}{n\quad \pi} \right)\left( {{f_{k + 1}\sin \quad \frac{2n\quad \pi}{L}\left( {k + 1} \right)} - {f_{k}\sin \quad \frac{2n\quad \pi}{L}k}} \right)} - {\left( \frac{L}{2n^{2}\pi^{2}} \right)\left( {f_{k + 1} - f_{k}} \right)\left( {{\cos \quad \frac{{2\quad n\quad \pi}\quad}{L}\left( {k + 1} \right)} - {\cos \quad \frac{2\quad n\quad \pi}{L}k}} \right)}}}\left( {{n = 1},2,\ldots}\quad \right)\quad {b_{n}^{\prime} = {{\sum\limits_{j = {k - L + 1}}^{k - 1}b_{n}^{j}} - {\left( \frac{1}{n\quad \pi} \right)\left( {{f_{k + 1}\cos \quad \frac{2n\quad \pi}{L}\left( {k + 1} \right)} - {f_{k}\cos \quad \frac{2n\quad \pi}{L}k}} \right)} - {\left( \frac{L}{2n^{2}\pi^{2}} \right)\left( {f_{k + 1} - f_{k}} \right)\left( {{\sin \quad \frac{{2\quad n\quad \pi}\quad}{L}\left( {k + 1} \right)} - {\sin \quad \frac{2\quad n\quad \pi}{L}k}} \right)}}}\left( {{n = 1},2,\ldots}\quad \right)}\quad} & (21)\end{matrix}$

[0068] More frame data can be used for interpolation of a higher order.

EXAMPLE 3

[0069] In Examples 1 and 2, a response time of the fluorescent materialis not considered. However, if the response time of the fluorescentmaterial is long, a frequency response of human eyes is substantiallydeteriorated. Therefore, the adjustment is performed in order todecrease the value of ξ_(n) in a high order. In general, the responsespeed of the fluorescent material depends on a color, so it is desirablethat the value of ξ_(n) is varied depending on a color.

EXAMPLE 4

[0070] In Examples 1 and 2, Fourier component in the period of pluralframes is considered. However, it is possible to consider Fouriercomponent within one frame, i.e., in the case where L=1. In this casetoo, a light emission pattern is selected so that the light emissionwaveform in the frame becomes smooth. Therefore, the state of lowluminance level is prevented from lasting long, so that pseudo contoursand flickers can be suppressed. The light emission pattern is determinedonly from the display luminance data of the frame, so the correspondenttable becomes compact.

EXAMPLE 5

[0071] The period for considering Fourier component is not necessarilyconstant. If the luminance level or the gradation level alters rapidly,a deviation of the time axis direction distribution of the lightemission intensity in the frame, for example, is hardly sensed by humaneyes as an abnormal display. Therefore, it is possible to determine thelight emission pattern, for example, by setting L to a value of two ormore normally, and by setting L to a value of “1” if the difference tothe luminance level or the gradation level of the adjacent frame islarge to a certain extent.

EXAMPLE 6

[0072] The subframe expression can be optimized also in the case wherethe frame period of the display device 100 (the length of the displayframe period) is different from the frame period of the frame data Dfthat is the original image (the transferring period of the originalframe). In this case, the target light emission waveform is defined asshown in FIG. 8 or FIG. 9 for evaluating an error. In this case, theunit of the period of Fourier expansion can be the frame period of thedisplay frame or the frame period of the original frame.

[0073] If the frame period of the display frame is adopted as the unit,f(t) is defined in accordance with display data. If the frame period ofthe original frame is adopted as the unit, subframes within one originalframe may be redefined as a set of subframes in the frame.

EXAMPLE 7

[0074] If the display device has a structure in which subframe data (alight emission pattern) are received and display is performed inaccordance with the received data, the subframe data can be generatedbeforehand from gradation data of an image, so as to be inputted intothe display device. In this way, the display device is not required todetermine the light emission pattern, and the circuit structure can besimplified. It is also possible to memorize such light emission patterndata in another memory device, and to reproduce the data in the displaydevice at any time.

[0075] In addition, this display device can be a semimanufacturedproduct (a plasma display module) that is combined with another modulesuch as an interface circuit to be a final product. Thus, a manufacturerof the final product can freely coordinate the method of determining thelight emission pattern, so that the flexibility of design can beincreased.

[0076] Moreover, in order to control power consumption of the displaydevice, it is desirable to calculate data of display load data of eachframe beforehand and to input them together for saving time and effortof calculating gradation data from light emission pattern data in thedisplay device.

[0077] According to the present invention, selection of a subframeexpression for reducing pseudo contours can be systematized and thesubframe expression can be optimized automatically.

[0078] While the presently preferred embodiments of the presentinvention have been shown and described, it will be understood that thepresent invention is not limited thereto, and that various changes andmodifications may be made by those skilled in the art without departingfrom the scope of the invention as set forth in the appended claims.

What is claimed is:
 1. A data conversion method for displaying an image,comprising conversion of original frame data indicating gradation of apixel into display frame data defining a light emission timing of adisplay element in a display frame period, the conversion including thesteps of; determining a light emission waveform in accordance withdisplay frame data of plural frames containing the current frame and theprevious frame; performing Fourier expansion of an error between thedetermined light emission waveform and a target light emission waveformdefined by the original frame data corresponding to the determined lightemission waveform; and setting the display frame data of the currentframe so that a sum of error components with weights that are obtainedby weighting each Fourier component.
 2. The data conversion methodaccording to claim 1, wherein the weight of each Fourier component isset individually for each light emission color of a display element. 3.The data conversion method according to claim 1, wherein the weight ofFourier component of a frequency above a flicker frequency is set to“0”.
 4. The data conversion method according to claim 1, wherein thedisplay frame period is different from the original frame period.
 5. Thedata conversion method according to claim 4, wherein the Fourierexpansion is performed for each time range having a unit of the displayframe period.
 6. The data conversion method according to claim 4,wherein the Fourier expansion is performed for each time range having aunit of the original frame period.
 7. The data conversion methodaccording to claim 1, wherein the target light emission waveform is aninterpolation waveform obtained by linear approximation of a transitionof discrete target light emission values in each original frame.
 8. Adata conversion method for displaying an image, comprising conversion oforiginal frame data indicating gradation of a pixel into display framedata defining a light emission timing of a display element in a displayframe period, the conversion including the steps of; performing Fourierexpansion of an error between a gradation waveform indicating atransition of gradation to be displayed and a target gradation waveform,an error with weight obtained by setting weight to each Fouriercomponent being small; performing Fourier expansion of an error betweena gradation waveform indicating a gradation transition defined bydisplay frame data of plural frames containing the current frame and theprevious frame and a target gradation waveform defined by original framedata corresponding to the gradation waveform; and setting the displayframe data of the current frame so that a sum of error components withweight that are obtained by weighting each Fourier component
 9. The dataconversion method according to claim 8, wherein the weight of eachFourier component is set individually for each light emission color of adisplay element.
 10. The data conversion method according to claim 8,wherein the weight of Fourier component of a frequency above a flickerfrequency is set to “0”.
 11. The data conversion method according toclaim 8, wherein the display frame period is different from the originalframe period.
 12. The data conversion method according to claim 11,wherein the Fourier expansion is performed for each time range having aunit of the display frame period.
 13. The data conversion methodaccording to claim 11, wherein the Fourier expansion is performed foreach time range having a unit of the original frame period.
 14. The dataconversion method according to claim 8, wherein the target gradationwaveform is an interpolation waveform obtained by linear approximationof a transition of discrete target gradation values in each originalframe.
 15. A display device expressing gradation of original frame databy controlling a light emission timing of a display element inaccordance with display frame data, the device comprising: an originalframe memory for memorizing original frame data of at least one frame; adisplay frame memory for memorizing display frame data of at least oneframe; a data converting circuit for outputting data corresponding to aninput data value as display frame data of the n-th frame, responding toan input of original frame data of the n-th frame, original frame dataof at least (n−1)th frame from the original frame memory and displayframe data of at least (n−1)th frame from the display frame memory,wherein the display frame data outputted by the data converting circuitare prepared by the data conversion method of claim
 1. 16. A displaydevice expressing gradation of original frame data by controlling alight emission timing of a display element in accordance with displayframe data, the device comprising: an original frame memory formemorizing original frame data of at least one frame; a display framememory for memorizing display frame data of at least one frame; a dataconverting circuit for outputting data corresponding to an input datavalue as display frame data of the n-th frame, responding to an input oforiginal frame data of the n-th frame, original frame data of at least(n−1)th frame from the original frame memory and display frame data ofat least (n−1)th frame from the display frame memory, wherein thedisplay frame data outputted by the data converting circuit are preparedby the data conversion method of claim 8.